Among fuel cells generating electric power utilizing an electrochemical reaction between hydrogen and oxygen, polymer electrolyte fuel cells are known. A polymer electrolyte fuel cell of this type has a stack which is constituted by a plurality of stacked cells. Each cell constituting the stack has an anode (fuel electrode) and a cathode (air electrode), and a solid polymer electrolyte membrane having a sulfonic acid group as an ion exchange group is disposed between the anode and the cathode.
A fuel gas (hydrogen-enriched reformed hydrogen obtained by reforming hydrogen gas or hydrocarbon) is supplied to the anode, while an oxidant gas (e.g., air) that contains oxygen as an oxidant is supplied to the cathode. Upon the supply of the fuel gas to the anode, hydrogen contained in the fuel gas reacts with a catalyst in a catalyst layer of the anode, resulting in the generation of hydrogen ions. The generated hydrogen ions pass through the solid polymer electrolyte membrane and electrically react with oxygen in the cathode. Through this electrochemical reaction, electric power is generated.
In a fuel cell system that utilizes a polymer electrolyte fuel cell as a power source, if the system stops the operation, the temperature of the fuel cell decreases, and the water within the fuel cell, which has been in hot and humid conditions until then, would condense to form dew drops, or freeze. In particular, when the temperature of the fuel cell is below zero, the water generated through the power-generation reaction freezes on the surface of the electrode, which would interfere with the supply of oxygen and inhibit the power-generation reaction.
So, when starting the system at a temperature below zero, a warm-up operation is performed in which an amount of oxidant gas supplied to the fuel cell is reduced to increase an amount of heat generation (see Patent Document 1 below). Also, in order to reduce problems that would occur when the fuel cell has a negative voltage with insufficient hydrogen gas during such a warm-up operation, a fuel cell system as described in Patent Document 2 below has been proposed.
In the technique described in Patent Document 2, a fuel cell system is controlled using a flowchart illustrated in FIG. 2 of the document. According to the flowchart in FIG. 2 of Patent Document 2, a warm-up operation is performed if the fuel cell is at a temperature of 0° C. or lower, and stopped if the temperature goes above 0° C.